The present invention relates to the field of packaging semiconductor devices, and more particularly to packaging light emitting diodes.
Light emitting devices (LEDS) such as light emitting diodes are often packaged within leadframe packages. A leadframe package typically includes an LED connected to thin metal leads where the LED and most of the leads are completely encapsulated within a plastic body. A part of the plastic body defines a lens. A portion of the leads connected to the LED extends outside the plastic body. The metal leads of the leadframe package serve as the conduit to supply the LED with electrical power and, at the same time, may act to draw heat away from the LED. Heat is generated by the LED when power is applied to the LED to produce light. The portion of the leads that extend out from the package body connects to circuits external to the leadframe package.
Some of the heat generated by the LED is dissipated by the plastic package body; however, most of the heat is drawn away from the LED via the metal components of the package. The metal leads are typically very thin and have a small cross section. For this reason, capacity of the metal leads to remove heat from the LED is limited. This limits the amount of power that can be sent to the LED. This, in turn, limits the amount of light that can be generated by the LED.
To increase the capacity of an LED package to dissipate heat, various designs are used in the industry; however, each of these designs results in LED packages with limited heat dissipation capacities while increasing the complexity and the costs of manufacturing the LED packages. For example, in one approach, LED packages are designed to include the LED within a cavity of a heatsink slug. Then, the heatsink slug is surrounded by a plastic body except for its bottom surface. For example, some LUXEON™ LED packages by Lumileds Lighting, LLC embodies such a design. Here, the heatsink slug increases the capacity of the LED package to dissipate heat; however, LED packages of this design are relatively difficult and costly to manufacture. Further, the heat dissipation is limited because of its limited exposed surface (the bottom surface only).
In another LED package design, the leads of the leadframe are extended (in various shapes and configurations) beyond the immediate edge of the LED package body. This increases the surface area of the portions of the leads exposed to the surrounding air. The increased exposed surface area of the extended leads increases the capacity of the LED package to dissipate heat; however, the extended leads increase the size of the LED package requiring relatively large area on a circuit board. Circuit board area is a scarce and costly factor in many applications.
Another undesirable aspect of the current leadframe package designs relates to problems associated with thermal expansion of the package. When heat is generated, the LED package experiences thermal expansion. Each of the parts of the LED package has a different coefficient of thermal expansion (CTE). For example, the CTE of the LED, the CTE of the package body, the CTE of the leads, and the CTE of lens are different from each other. For this reason, when heated, each of these parts experience different degrees of thermal expansion resulting in mechanical stresses between the parts of the package thereby adversely affecting its reliability.
Consequently, there remains a need for an improved LED package that overcomes or alleviates one or more of the shortcomings of the prior art packages.